The present invention relates to improved optical solar reflectors, and in particular to improved materials and processes for optical solar reflectors.
Optical solar reflectors (OSRs) are used in a variety of applications, including space applications, and are often referred to as heat reflectors or thermal control coatings. The function of optical solar reflectors (OSRs) on spacecraft is to minimize thermal absorption and thermal variations in sensitive electronic devices caused by the devices themselves and by solar radiation. Keeping the electronics cool is critical to performance where the power that can be dissipated is dependent on the junction temperature of the active devices. Hence for every degree that the electronic devices can be cooled by emitting heat from the surface or every degree that they are not heated from the sun, the electronic devices can put out several more watts and still remain under the critical junction temperature.
For space applications, OSRs must not absorb in the solar spectrum, typically 200 to 2500 nm, but must absorb and emit from 2.5 to 25 microns, which corresponds to the wavelength of heat radiation generated from electronic equipment the OSRs encompass. This leads to the requirement that must be met by the materials from which an OSR is constructed that the material must have a low ratio of absorption to emissivity. This ratio is defined as xcex1E, where xcex1 is the absorption coefficient, and E is the emissivity of the material. Additionally, the material from which an OSR is constructed remains stable even in the radiation conditions found in space. This greatly limits the number of materials that can be used.
Prior art OSRs have been constructed by coating a material such as glass, which is known to have low solar absorbency, with a highly reflective material such as Ag or Al so that the solar energy passes through the glass and reflects off the Ag. These silver-coated thin glass sheets are then manually glued to either carbon composite or aluminum alloy substrates. One disadvantage of this construction technique is the relatively high cost of the OSR, due to the extensive labor involved and the high breakage of parts in manual handling.
Another reason that glass has been used in prior art OSRs is because it has characteristic absorbency in the IR region to emit the heat generated from the substrate. The prior art OSR is such a structure which is typically a 3xe2x80x3xc3x973xe2x80x3xc3x970.002xe2x80x3 piece of float glass onto which is deposited a silver layer for reflectance and a chromium layer for adhesion. The size and thickness of the prior art OSR is limited by the strength of the float glass. This thickness is a significant weight penalty for the spacecraft. Attempts to make thinner OSRs using prior art construction have been unsuccessful because they become very brittle so that they cannot be handled and the emissivity drops off causing poor performance. Even at the current thickness, they are very brittle and a large percentage are broken in handling and trying to cement them to the space craft substrate (tile). The current area of 3xe2x80x3xc3x973xe2x80x3 is also a major limitation since the average spacecraft will require a very large number of such pieces. In addition, the absorbency of the float glass in the solar region is high due to the additives used to provide ductility to the glass. For example, CeO2, TiO2, and ZnO may be added, but these materials absorb energy in the UV spectrum. In addition, the absorbency of the float glass increases in space due to chemical changes from radiation.
Past efforts to improve OSRs have involved attempts to make thin free standing pure SiO2 to overcome these limitations; but this material was too brittle to be handled in the thickness involved. Attempts have also been reported to deposit pure SiO2 onto substrates such as aluminum by CVD processes, but the high temperatures used in those processes resulted in large thermal expansion mismatches, which caused delamination. An alternative approach of anodizing aluminum to alumina was tried, however the trace impurities in the substrate caused color centers to develop in space.
Thus, a need arises for optical solar reflectors that are constructed out of new materials and manufactured using different processes, in order to improve performance and reduce cost.
Two additional problems also arise. First, optical solar reflectors (OSRs) undergo severe thermal cycle requirements. These thermal cycles put enormous strain on the interfaces between layers of the OSR, which is generally due to differences in the coefficients of thermal expansion (CTE) of the various layers. This CTE mismatch, as well as poor chemical bonding between layers, or changes in chemical bonding during cycling, leads to failure of the OSR due to delamination of one or more layers. A need arises for an OSR having improved thermal cycle performance.
Secondly, the xcex1/E performance of an OSR is adversely affected by surface roughness of the reflector. A need arises for a technique by which surface roughness can be improved.
The present invention is an optical solar reflector, and method of production thereof, which provides excellent performance, reduced cost, and reduced breakage due to reduced manual handling. The optical solar reflector of the: present invention comprises a substrate, a bond layer coating the substrate, a reflective layer coating the bond layer, and a radiative layer coating the reflective layer.
Preferably, the radiative layer comprises SiO2, Si3N4, or SiOxNy, has low absorbency of electromagnetic radiation having wavelengths in the range of approximately 200 nm to approximately 2500 nm, high absorbency and emissivity of electromagnetic radiation having wavelengths in the range of approximately 2.5 xcexcm to approximately 25 xcexcm, and more specifically at approximately 8.6 xcexcm, a thickness of approximately 10-25 xcexcm, and more specifically approximately 16 xcexcm, and is deposited by a plasma enhanced chemical vapor deposition process.
Preferably, the reflective layer comprises aluminum or silver and has a thickness of approximately 50-1000 nm, and more specifically approximately 300 nm.
Preferably, the bond layer comprises chromium, titanium, or titanium-tungsten and has a thickness of approximately 10 nm, and more specifically approximately 20 nm.
Preferably, the substrate comprises aluminum, aluminum alloys, polyimide, carbon-filled polyimide, and carbon composites.
The optical solar reflector of the present invention may further comprise a barrier layer between the reflective layer and the radiative layer. Preferably, the barrier layer comprises MgF2, has a thickness in the range of approximately 14 nm to 40 nm, and improves adhesion between the reflective layer and the radiative layer during thermal cycling.
The optical solar reflector of the present invention may further comprise a surface-leveling layer between the substrate and the bond layer. Preferably, the surface-leveling layer comprises a silicone hardcoat material, has a thickness of approximately 5 xcexcm, and improves surface smoothness of the substrate.
The radiative layer may have a modulated refractive index profile. The modulated refractive index profile of the radiative layer may have a plurality of parameters, including peak-to-peak refractive index excursion, period of modulation in optical thickness, and number of modulation cycles. The parameters of the modulated refractive index profile of the radiative layer may control amplitude, bandwidth, and wavelength of rejection bands of the radiative layer.